1. Field of Invention
The invention relates generally to instruments and methods for examining and treating the eye and specifically to various kinds of ophthalmoscopes equipped with laser sources for the purpose of applying a therapeutic laser beam to the retina of an eye.
2. Description of Prior Art
Ophthalmoscopes, exemplified by the biomicroscope, are combined with a non-scanning therapeutic laser source for the purpose of retinal photocoagulation. Usually, a contact glass is placed on the cornea to be able to view the retina with the biomicroscope, and a mirror is used for reflecting the therapeutic laser beam onto the desired retinal location through a small part of the pupillary area. Importantly, the retina is illuminated with a strong visible light, and it is observed through different parts of the pupillary area to avoid reflexes, an arrangement that is known as Gullstrand's principle of ophthalmoscopy. Such optical configuration makes the art of precise focusing of a therapeutic laser beam on the retina more difficult. This is invariably the case in the presence of wavefront aberrations of the eye optics, a small pupil diameter, or a large diameter therapeutic laser beam. Vignetting of the external laser beam can possibly harm the anterior ocular structures.
To overcome the previous problems, U.S. Pat. No. 4,213,678, issued Sep. 29, 1980 to Pomerantzeff et al. discloses a co-pupillary scanning laser ophthalmoscope for the purpose of diagnosing and treating retinal disease using two different intensity levels of the scanning laser beam. One intensity range can be used for monochromatic imaging and angiography while a much higher level of the same laser beam or a different coaxial scanning laser beam is used for retinal photocoagulation. This novel approach however is not ideal because of the difficulties in implementing safety controls for such a scanning therapeutic laser beam, the difficulty in modulating the scanning laser beam over a range from non-coagulating to coagulating energies at video bandwidth, and the non-thermal complications of a high intensity pulsed laser beam in the nanosecond domain combined with an inappropriate duty cycle. Nevertheless, such a temporally modulated microphotocoagulation as proposed before by Birngruber and Roider, is useful to restrict the impact of the therapeutic application to the retinal pigment epithelium. However, an appropriate duty cycle is necessary and this cannot be achieved with a scanning therapeutic laser source as previously proposed; hence the necessity of an external non-scanning pulsed therapeutic source for this purpose.
Small, minimal intensity applications that are not pulsed and of longer duration, can more selectively target the photoreceptors through a combination of photochemical and thermal mechanisms of injury. These applications save functional retinal tissue in between them, and also these smaller spots can take a variability in absorption more easily into account. However, classic photocoagulating ophthalmoscopes have been limited in usefulness when such minimal intensity threshold laser is applied. One reason is that the anatomical changes caused by the therapeutic laser are often very difficult to visualize during the application in the presence of the photocoagulating light. The critical endpoint of such laser applications is often exceeded because the surgeon, upon recognizing the minimal anatomical changes within the retina, is also handicapped by a substantial human reaction time delay before s/he can interrupt the therapeutic laser. In addition, it is difficult to permanently document selective therapeutic laser applications on the retinal image because both threshold and pulsed applications are typically not visible a short time after the delivery.
U.S. Pat. Nos. 5,923,399, 5,943,177 and 5,892,569 to Van de Velde address these problems and they describe different embodiments of a confocal scanning laser ophthalmoscope that is optimized for delivering selective therapeutic laser of various nature to the retina. This includes temporally modulated applications, small threshold continuous applications and applications that use a photosensitizer drug. The latter method is called photodynamic therapy, similar to transpupillary thermal therapy without the dye injection, and typically uses long duration, larger circular applications of laser light that is preferentially absorbed by the photosensitizer drug. It aims at selectively closing abnormal small blood vessels within or underneath the retina.